There was water, as other studies have shown, but no natural nuclear reactors it’s very much a modern world, just with less oxygen.’ ‘The data indicates that the first 500 million years of Earth’s history was not the hellish environment people typically describe. ‘The fact that the composition of Hadean zircons is similar to that of primitive meteorites means nuclear reactors did not influence these zircons’ composition,’ Tissot clarifies.
Tissot’s team found δ 238U variations of -0.60 to -0.12‰, nothing near the +24 to +475‰ found near the site of the only known natural nuclear reactor in the Central African state of Gabon. Natural reactors would have increased Earth’s temperature and radiation levels, and with a largely molten surface and an atmosphere of poisonous gas, even basic microorganisms would not have survived. Previous research had shown that early Earth had elevated levels of 235U, high above the threshold required for spontaneous nuclear fission so theory predicted that numerous natural nuclear reactors existed.
The team applied their technique to 31 Hadean zircon samples and were surprised to find the uranium isotope levels were similar to those found in primitive meteorites. There is a lot of potential in achieving this challenging measurement.’ Tissot explains, ‘because the uranium isotope system allows us to probe for the presence of natural nuclear reactors, redox conditions and geochronology, the idea of measuring uranium isotopes in single zircon crystals, particularly Hadean ones, is an important target. Since samples must be diluted for mass spectrometric analysis, the team needed a way to handle liquid volumes 10 times lower than usual so that the small uranium samples obtained from the zircon were elevated to detectable concentrations. The size of these samples, however, has limited studies on them to certain elements.įrançois Tissot at the California Institute of Technology, US, and colleagues have developed a technique using multi-collector inductively coupled plasma-mass spectrometry that can analyse the uranium isotopes 238U and 235U within a sample. A sedimentary rock section in the Jack Hills, 800km north of Perth, in Western Australia, contains zircons eroded from rocks as old as 4.3 billion years, making these zircons the oldest recorded geological material on the planet. No rocks exist that are older than about 3.8 billion years so geochemical studies on the Hadean eon, Earth’s first 500 million years, revolve around incredibly durable zirconium silicate mineral grains called zircons. Rocks can provide clues for researchers trying to reconstruct Earth’s history. So say researchers who have developed a new analytical procedure to measure uranium isotope levels in tiny crystal structures that are over 4 billion years old. Earth’s crust was actually more like it is today than previously thought. There is no evidence that natural nuclear fission reactors were widespread on the early Earth. This grain contains primary inclusions of quartz (red), rutile and muscovite (black)
Colour shades represent the difference in elemental composition. Cathodoluminescence image of a zircon crystal from the Jack Hills conglomerate, similar to the ones studied in this work.
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